1. Field of the Invention
The present invention generally relates to control systems for moving a movable member under sequentially applied driving pressures and particularly to such control systems wherein the movement between end points is initially accelerated, then decelerated, and then again accelerated.
2. Description of the Prior Art
Prior art systems used to move movable members, such as machine safety curtains and press bolsters, between predetermined travel points involved driving the member under a constantly applied force during the length of its travel. This involved a constant acceleration of the movable member and produced shocks when the member was stopped at the end points of the travel length. To eliminate these shocks shock absorbers were added at the end points which were springs or hydraulic mechanisms. In either event the shock absorbers had to be carefully sized for the particular mass and speed of the movable member. Further, the shock absorbers tended to wear out and fail under the constant pounding from the movable member and required costly maintenance and replacement.
Some fluid power systems are known which mechanically or time-delay actuate flow control valves or restrictions in the system to throttle the driving device and decelerate the motion of the driven member. However, such devices are operated with incompressible fluids such as liquids and are not readily applied to compressible fluids such as air. Attempts to adapt such devices to compressible fluids causes non-uniform deceleration so that surging and even rebound at the end of driven member travel may result.
Examples of the forementioned devices may be found in U.S. Pat. Nos. 3,803,943; 3,264,942; 2,916,879 and the reader is referred thereto for further detail of structure and operation.